The present invention relates to electrophoretic displays, particularly encapsulated electrophoretic displays, and to materials useful in fabricating such displays.
Electrophoretic displays have been the subject of intense research and development for a number of years. Electrophoretic displays have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays has, to date, prevented their widespread usage.
The recent invention of encapsulated electrophoretic displays solves many of these problems and offers additional advantages compared to liquid crystal displays. Some added advantages are the ability to print or coat the display material on a wide variety of flexible and rigid substrates. The clustering and settling problems, which plagued prior art electrophoretic displays and resulted in inadequate lifetimes for the displays are now overcome.
The purpose of this disclosure is to describe electrophoretic displays, especially encapsulated electrophoretic displays, and classes of materials, as well as some specific materials, which should be useful in their construction.
The successful construction of an encapsulated electrophoretic display requires the proper interaction of several different types of materials and processes. Materials such as a polymeric binder, a capsule membrane, and the electrophoretic particles and fluid must all be chemically compatible. The capsule membranes may engage in useful surface interactions with the electrophoretic particles, or may act as an inert physical boundary between the fluid and the binder. Polymer binders may set as adhesives between capsule membranes and electrode surfaces.
In some cases, a separate encapsulation step of the process is not necessary. The electrophoretic fluid may be directly dispersed or emulsified into the binder (or a precursor to the binder material) to form what may be called a xe2x80x9cpolymer-dispersed electrophoretic displayxe2x80x9d. In such displays, the individual electrophoretic phases may be referred to as capsules or microcapsules even though no capsule membrane is present. Such polymer-dispersed electrophoretic displays are considered to be subsets of encapsulated electrophoretic displays.
In an encapsulated electrophoretic display, the binder material surrounds the capsules and separates the two bounding electrodes. This binder material must be compatible with the capsule and bounding electrodes and must possess properties that allow for facile printing or coating. It may also possess barrier properties for water, oxygen, ultraviolet light, the electrophoretic fluid, or other materials. Further, it may contain surfactants and cross-linking agents, which could aid in coating or durability. The polymer-dispersed electrophoretic display may be of the emulsion or phase separation type.
The present invention provides electrophoretic displays, particularly encapsulated electrophoretic displays, and materials for use in such displays. The capsules may be spherical or non-spherical in shape. In electrophoretic displays, at least some of the particles are moved or rotated by application of electric fields. The electric field may be an alternating-current field or a direct-current field. The electric field may be created by at least one pair of electrodes disposed adjacent a binder material containing the particles. The particles may be absorbing pigments, scattering pigments or luminescent particles, for example. The particles may be made up of some combination of dye, pigment, polymer.
Such displays may also include, for example, one type of particle that retroreflects, or substantially retroreflects, light and another type that absorbs light. Application of an electric field may cause the particles in an encapsulated display to orient so that the capsule retroreflects, or substantially retroreflects, light. Application of another electric field may cause the particles to orient so that the capsule absorbs, or does not retroreflect, light. A display may also include a reflective substrate, so that orientation of one type of particle in a particular pattern causes light to pass through the capsule to the substrate, which reflects light. Orientation of a second type of particle in a particular pattern causes the capsule to absorb, or otherwise not reflect, light. Types of retroreflective and reflective materials that may be used in constructing a retroreflective or reflective substrates, respectively, include glass spheres and diffractive reflecting layers.
Another type of display has particles of differing colors. Such a display has at least two, and preferably at least, three different species of particles, with each type of particle having a different electrophoretic mobility. The different electrophoretic mobilities provide the particles with substantially non-overlapping electrophoretic mobilities, so that application of different electric fields causes different subsets of the colored particles to be viewed at the surface of the capsule.
Another type of display includes luminescent particles and a visible light-blocking medium, which may contain light-absorbing particles or dyes. Application of different electric fields may cause the particles to luminesce selectively or uniformly at the front (eyes see a bright pixel) or rear (fluid absorbs radiation) of the capsule. Application of different electric fields may cause either the luminescent particles or the light-blocking particles to rise to the capsule surface, resulting in either a light or a dark appearance to the capsule, respectively.
In another type of electrophoretic display, the particles may themselves be encapsulated pigments, dyes, pigment dispersions, dye solutions, or a combination of any of these. These particles are dispersed in a suspending fluid and are then encapsulated into capsules in a binder. The particles may be dispersed within a suspending fluid and may each contain a plurality of solid particles or a dye or both. The suspending fluid can be a single fluid or a mixture of two or more fluids. In one embodiment, the particles may have a diameter from between about 10 nm and about 5 xcexcm, whereas the capsules may have a diameter from between about 5 xcexcm and about 200 xcexcm. In another embodiment, the particles may have a flexible outer surface or may be a polymeric layer surrounding a dye or dye solution.
The advantage of this system is that known emulsification or encapsulation techniques can be used to make improved particles, with better control of absorbance, optical properties, charge, mobility, shape, size, density, surface chemistry, stability, and processibility. There are vast numbers of dyes and/or particles and liquids of all polarities that can be used to gain a high level of control over the optical properties of the system. It is possible to create particles which are capsules containing dyes and/or particles in order to obtain properties difficult to achieve with pigments. The present invention relates to these encapsulated electrophoretic displays and the materials, such as dyes, pigments, binder, etc. that may be useful in their construction.
Encapsulated electrophoretic displays may include two or more different types of particles. Such displays may include, for example, displays containing a plurality of anisotropic particles and a plurality of second particles in a suspending fluid. Application of a first electric field may cause the anisotropic particles to assume a specific orientation and present an optical property. Application of a second electric field may then cause the plurality of second particles to translate, thereby disorienting the anisotropic particles and disturbing the optical property. Alternatively, the orientation of the anisotropic particles may allow easier translation of the plurality of second particles. The particles may have a refractive index that substantially matches the refractive index of the suspending fluid.
Finally, an encapsulated display may comprise an electro-osmotic display. Such a display may comprise capsules containing a refractive index matching fluid, that moves within the capsule to create a homogeneous capsule upon application of an electric field. The capsule may also contain a porous internal material, such as an alkylcellulose, that swells upon movement of the refractive index matching fluid within the capsule. An electro-osmotic display may also include two or more immiscible fluids, that move within the capsule to create a different optical property upon application of an electric field. The optical effect may result from a planar index mismatch or a non-planar index mismatch.
Materials for use in creating electrophoretic displays relate to the types of materials, including, but not limited to, particles, dyes, suspending fluids, and binders used in fabricating the displays. In one embodiment, types of particles that may be used to fabricate suspended particle displays include scattering pigments, absorbing pigments and luminescent particles. Such particles may also be transparent. Preferred particles include titania, which may be coated in one or two layers with a metal oxide, such as aluminum oxide or silicon oxide, for example. Such particles may also be retroreflective or have a reflective coating. Such particles may be constructed as corner cubes. Luminescent particles may include, for example, zinc sulfide particles. The zinc sulfide particles may also be encapsulated with an insulative coating to reduce electrical conduction. Light-blocking or absorbing particles may include, for example, dyes or pigments.
A suspending (i.e., electrophoretic) fluid may be a high resistivity fluid. The suspending fluid may be a single fluid, or it may be a mixture of two or more fluids. The suspending fluid, whether a single fluid or a mixture of fluids, may have its density substantially matched to that of the particles within the capsule. The suspending fluid may be a halogenated hydrocarbon, such as tetrachloroethylene, for example. The halogenated hydrocarbon may also be a low molecular weight polymer. One such low molecular weight polymer is poly(chlorotrifluoroethylene). The degree of polymerization for this polymer may be from about 2 to about 10.
Types of dyes for use in electrophoretic displays are commonly known in the art. They may be soluble in the suspending fluid. These dyes may further be part of a polymeric chain. Dyes may be polymerized by thermal, photochemical, and chemical diffusion processes. Single dyes or mixtures of dyes may also be used.
Furthermore, capsules may be formed in, or later dispersed in, a binder. Materials for use as binders include water-soluble polymers, water-dispersed polymers, oil-soluble polymers, thermoset polymers, thermoplastic polymers, and uv- or radiation-cured polymers.